SYNOPSIS

DESCRIPTION

As of Linux 2.2,
the power of the superuser (root) has been partitioned into
a set of discrete capabilities.
Each thread has a set of effective capabilities identifying
which capabilities (if any) it may currently exercise.
Each thread also has a set of inheritable capabilities that may be
passed through an
execve(2)
call, and a set of permitted capabilities
that it can make effective or inheritable.

These two functions are the raw kernel interface for getting and
setting thread capabilities.
Not only are these system calls specific to Linux,
but the kernel API is likely to change and use of
these functions (in particular the format of the
cap_user_*_t
types) is subject to extension with each kernel revision,
but old programs will keep working.

The portable interfaces are
cap_set_proc(3)
and
cap_get_proc(3);
if possible you should use those interfaces in applications.
If you wish to use the Linux extensions in applications, you should
use the easier-to-use interfaces
capsetp(3)
and
capgetp(3).

Current details

Now that you have been warned, some current kernel details.
The structures are defined as follows.

effective, permitted, inheritable
are bitmasks of the capabilities defined in
capability(7).
Note the
CAP_*
values are bit indexes and need to be bit-shifted before ORing into
the bit fields.
To define the structures for passing to the system call you have to use the
struct __user_cap_header_struct
and
struct __user_cap_data_struct
names because the typedefs are only pointers.

Kernels prior to 2.6.25 prefer
32-bit capabilities with version
_LINUX_CAPABILITY_VERSION_1,
and kernels 2.6.25+ prefer 64-bit capabilities with version
_LINUX_CAPABILITY_VERSION_2.
Note, 64-bit capabilities use
datap[0]
and
datap[1],
whereas 32-bit capabilities only use
datap[0].

Another change affecting the behavior of these system calls is kernel
support for file capabilities (VFS capability support).
This support is currently a compile time option (added in kernel 2.6.24).

For
capget()
calls, one can probe the capabilities of any process by specifying its
process ID with the
hdrp->pid
field value.

With VFS Capability Support

VFS Capability support creates a file-attribute method for adding
capabilities to privileged executables.
This privilege model obsoletes kernel support for one process
asynchronously setting the capabilities of another.
That is, with VFS support, for
capset()
calls the only permitted values for
hdrp->pid
are 0 or
getpid(2),
which are equivalent.

Without VFS Capability Support

When the kernel does not support VFS capabilities,
capset()
calls can operate on the capabilities of the thread specified by the
pid
field of
hdrp
when that is nonzero, or on the capabilities of the calling thread if
pid
is 0.
If
pid
refers to a single-threaded process, then
pid
can be specified as a traditional process ID;
operating on a thread of a multithreaded process requires a thread ID
of the type returned by
gettid(2).
For
capset(),
pid
can also be: -1, meaning perform the change on all threads except the
caller and
init(8);
or a value less than -1, in which case the change is applied
to all members of the process group whose ID is -pid.

RETURN VALUE

On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.

The calls will fail with the error
EINVAL,
and set the
version
field of
hdrp
to the kernel preferred value of
_LINUX_CAPABILITY_VERSION_?
when an unsupported
version
value is specified.
In this way, one can probe what the current
preferred capability revision is.

ERRORS

EFAULT

Bad memory address.
hdrp
must not be NULL.
datap
may only be NULL when the user is trying to determine the preferred
capability version format supported by the kernel.

EINVAL

One of the arguments was invalid.

EPERM

An attempt was made to add a capability to the Permitted set, or to set
a capability in the Effective or Inheritable sets that is not in the
Permitted set.

EPERM

The caller attempted to use
capset()
to modify the capabilities of a thread other than itself,
but lacked sufficient privilege.
For kernels supporting VFS
capabilities, this is never permitted.
For kernels lacking VFS
support, the
CAP_SETPCAP
capability is required.
(A bug in kernels before 2.6.11 meant that this error could also
occur if a thread without this capability tried to change its
own capabilities by specifying the
pid
field as a nonzero value (i.e., the value returned by
getpid(2))
instead of 0.)